Fibre Channel is a 1 gigabit per second data transfer interface technology that maps several common transport protocols including IP and SCSI, allowing it to merge to high-speed I/O and networking functionality in a single connectivity technology. Fibre Channel is an open standard as defined by ANSI and OSI standards and operates over copper and fiber optic cabling at distances of up to 10 kilometers. ANSI ASC (Accredited Standards Committee) X3T11 is the primary committee responsible for Fibre Channel. It is unique in its support of multiple inter-operable topologies including point-to-point, arbitrated loop and switching and it offers several classes of service for network optimization. With its large packet sizes, Fibre Channel is ideal for storage, video, graphic and mass data transfer applications.
Fibre Channel Arbitrated loop was developed with peripheral connectivity in mind. It natively maps SCSI (as SCSI FCP), making it an ideal technology for high speed I/O connectivity. Native Fibre Channel Arbitrated Loop (FC-AL) disk drives allows storage applications to take full advantage of Fibre Channel's gigabit bandwidth, passing SCSI data directly onto the channel with access to multiple servers or nodes. FC-AL supports 127 node addressability and 10 km cabling ranges between nodes. Gigabit bandwidth and functionality also make Fibre Channel technology an attractive solution for server clustering.
Fibre Channel Arbitrated Loop (FC-AL) offers the highest overall performance and distance of any serial interface. Fibre Channel can transfer at 200 Mbytes/sec in full duplex mode over distances of 10 kilometers. Fibre Channel supports dual loop capability to provide a high resiliency environment. If a single loop is unavailable, the second loop continues operation. Fibre Channel hubs represent an additional level of control and resiliency. Hubs provide expansion flexibility where additional hosts and storage subsystems can be added with no disruption in the loop.
Arbitrated loop is implemented as a topology that takes the logic of switched topology and distributes it to all devices on the loop. This enables each device to use the loop as a point-to-point connection. Arbitrated loop works in a fashion where each device arbitrates for loop access, and once granted, has a dedicated connection between sender and receiver. The available bandwidth of the loop is shared between all devices. Since no switch is required to connect multiple devices, the cost per connection is significantly less.
When interconnecting Fibre-Channel Arbitrated Loop (FC-AL) enclosures together containing multiple FC devices, disruption of the loop may arise. For example, a cable may become detached from an enclosure or power may be inadvertently shut off or lost for a particular enclosure, an enclosure may need to be removed from the loop for servicing, etc. Under any of the above conditions, the loop is opened and the system becomes inoperable since each FC device in an FC-AL system acts as a repeater passing data around a loop unidirectionally. In Fibre Channel Arbitrated Loop environments, because each Fibre Channel node acts as a repeater for all other nodes that it is connected to, one failed node will bring the entire loop down.
Typically, an active hub or concentrator may be utilized into which each enclosure is connected. However, utilization of an active hub has several disadvantages. The central hub is a single point of failure which will bring down the entire loop if failure of the hub occurs. Additionally, the use of an active hub adds cost to the FC-AL, system.
An enhancement to this method of providing loop coherency in Fibre Channel systems is achieved by creating PORT A and PORT B loops. The PORT A loop is utilized exclusively until a failure condition at which time the traffic is switched to the PORT B loop. These loops are created by cabling two fully independent, physical loops between enclosures. Redundant cabling and hardware provide full data path redundancy.
Although a hub or concentrator will automatically bypass a problem port and avoid most faults, it represents a single point of failure. Redundancies can be built into concentrators or fully redundant cabling and concentrators can be used to work around this. A concentrator configuration may detrimentally add to the cost of the loop system.
None of the known techniques to provide loop integrity in a Fibre Channel Arbitrated Loop environment provide a loop system having scaleable architecture and no central point of failure at a low cost. Thus, there lies a need to provide an economical and scaleable method and apparatus for maintaining loop coherency without a central point of loop failure in Fibre Channel Arbitrated Loop environments.